1. Field of the Invention
This invention relates to a hydraulic transmission coupling apparatus for enabling four-wheel drive by distributing the driving force to the front and rear wheels through the hydraulic pressure generated by a hydraulic pump interposed between both wheels.
2. Description of Related Art
A four-wheel drive vehicle driven by the driving force of the engine transmitted to the front and rear wheels is in the limelight because of its capability of providing stability and comfort in running regardless of the road and running conditions. Four-wheel drive is enabled basically by directly connecting the front and rear wheels to the engine as a driving source. However, the four-wheel drive vehicle as above is followed by a problem that the drive is difficult for general drivers on account of peculiar behavior of the vehicle at the time of fast steering because of the so called "tight corner breaking phenomenon" arising from the inability to asborb a rotation speed difference between the front and rear wheels. Recently, therefore, a so-called "full-time four-wheel drive vehicle" equipped with a hydraulic transmission coupling apparatus which distributes the driving force to the front and rear wheels correspondingly to the rotation speed difference between both wheels while absorbing the pressure difference, has been most prevalent. An apparatus utilizing hydraulic pressure generated by an oil hydraulic pump has been developed as one of a plurality of hydraulic transmission coupling apparatuses.
This hydraulic transmission coupling apparatus is of such structure that a speed difference control part, which controls the speed difference between the front and rear wheels with the force corresponding thereto, is added to, for example, the differential gear unit disposed in the middle of a system for transmitting the force from the engine to the front and rear wheels. As a means for generating such controlling force as above at the speed difference control part, oil hydraulic pressure generated by the oil hydraulic pump is utilized.
The differential gear unit comprises a plurality of planet wheels revolving on a common orbital axis, rotating independently from each other, and adapted to mesh with a pair of differential gears respectively, the unit being a mechanical element for distributing the force inputted into the orbital axis to the output ends individually interlocked with the differential gears. Since the rotation speed difference caused between the output ends is absorbed by the rotation of the planet gears, force transmission to output ends having rotation speeds different from each other is possible. Accordingly, the provision of a differential gear unit (central differential gear unit) comprising a pair of differential gears interlocked with the front and rear wheels respectively and interlocking of the orbital axes of the planet gears with the engine ensures a four-wheel drive vehicle capable of absorbing the rotation speed difference between the front and rear wheels and of transmitting the driving force to both wheels. The differential gear unit, however, has a transmission characteristic such as distributing the driving force more to the output side of a high rotation speed than to the other when distributing the force to the output ends having rotation speeds different from each other. Thus, in the four-wheel drive vehicle as described above, when the wheels on any one of the front and rear sides are at an idle, a large part of the driving force of the engine is transmitted to the wheels on the idling side whereas, disadvantageously, only a small part to the wheels on the non-idling side requiring the driving force.
The above-mentioned speed difference control part is provided for applying the controlling force to a pair of differential gears in the differential gear unit corresponding to the degree of rotation speed difference between gears so as to secure an amount of the driving force to be distributed to the side of low rotation speed. The speed difference control part utilizing the oil hydraulic pressure generated by the oil hydraulic pump is provided with a casing rotating in association with the differential gears on one side, that is, the wheels on any one of the front and rear sides, a rotor contained in the casing and rotating in association with the differential gears on the other side, and a pump room between the casing and rotor. Relative rotations corresponding to the rotation speed difference between the front and rear wheels are caused between the casing and rotor and the hydraulic pressure increasing or decreasing corresponding to the number of relative rotations is generated in the pump room. The oil pressure thus generated acts upon the space between the casing and rotor for suppressing the aforesaid relative rotations. An amount of the driving force to be distributed to the front and rear wheels interlocking with the casing and rotor respectively is determined to be equalized through the abovesaid oil pressure. Thus, the disadvantage in the differential gear unit as described earlier is eliminated and four-wheel drive is made possible as desired.
In this speed difference control part, since the casing and rotor rotate together, it is difficult to supply hydraulic oil into the pump room from a tank fixedly disposed outside the casing and rotor. Therefore, an oil reserving part for reserving hydraulic oil is formed integrally with the casing while utilizing an annular room formed between a cylindrical surrounding member disposed to surround the casing and the periphery of the casing, and, an oil suction passage and an oil discharge passage communicating the oil reserving part with the pump room are formed in respective parts of the casing and rotor so that hydraulic oil reserved in the oil reserving part may be used while circulated between the oil reserving part and pump room. A quantity of hydraulic oil to be reserved in the oil reserving part is limited on account of the necessity to reduce the size of the hydraulic transmission coupling apparatus and the weight at the time of rotation thereof whereas pressure increase in the pump room every time of circulation and pressure decrease at the time of return of circulating oil to the oil reserving part are repeated. When the vehicle runs for a long time with a large difference in rotation speed between the front and rear wheels, for example, when running on the snowy road or mountain road, a temperature of hydraulic oil rises considerably high whereas, during parking in cold environments, the temperature falls considerably low because of contact of oil with the atmosphere. Such temperature variation causes volume variation of the hydraulic oil, which results in an internal pressure rise with the rise of oil temperature as well as an internal pressure fall with the fall of oil temperature in the oil reserving part. In the former case, outward leakage of hydraulic oil from the oil reserving part occurs and, in the latter case, the ingress of ambient air from the outside occurs, both cases interfering with the normal performance of the speed difference control part.
In the speed difference control part utilizing the oil pressure generated by the oil hydraulic pump, means to absorb the volume variation of hydraulic oil occurring with oil temperature variation is indispensable. Means to absorb the volume variation depending on the deformation of a diaphragm disposed in the oil reserving part is known (U.S. Pat. No. 3,393,583). This structure, however, suffers such problems that unrequired deformation of the diaphragm is caused by the action of centrifugal force following the rotation of the casing and, particularly, at the time of high speed running of the vehicle causing a strong centrifugal force, normal absorption of the volume variation is impossible.
For eliminating the influence of centrifugal force, the applicant of this invention have previously proposed a hydraulic transmission coupling apparatus wherein a piston member is disposed slidably in the axial length direction in a hollow space formed in the position of the axis of the rotary shaft of the rotor so as to introduce the internal pressure in the oil reserving part into one side of the piston member for absorbing the volume variation in hydraulic oil with the variation in the internal volume of the hollow space following sliding of the piston member, and, further, another hydraulic transmission coupling apparatus wherein the abovesaid surrounding member located outside the casing for forming the oil reserving part so that the internal volume of the oil reserving part may be directly varied with sliding of the surrounding member partly subjected to the internal pressure in the oil reserving part for absorbing the volume variation in the oil reserving part. These apparatuses are free of apprehension that the piston member and surrounding member engaged in absorption of the volume variation slide in the axial length direction and the radially acting centrifugal force exerts influence upon sliding, whereby the influence of centrifugal force is eliminated and adequate absorption of the volume variation is possible.
The structure of the former is subject to a limit to the design that a hollow space must be provided for the rotary shaft of the rotor. Further, because of sliding of the piston member performed in the hollow space formed in the position of the axial center thereof, an elongated sliding stroke is required for enabling sure absorption of the volume variation in a wide range from the presumed highest oil temperature to the lowest one, thereby hampering size reduction in the whole of the apparatus.
In the latter permitting sliding of the surrounding member, even a relatively short sliding stroke enables absorption of a large variation in volume and requires no increase in the axial length. However, variation in external shape of the hydraulic transmission coupling apparatus caused by sliding of the surrounding member imposes a limit that an installation space must be provided in consideration of the abovesaid variation in external shape when setting the hydraulic transmission coupling apparatus.